For video coding processes, several standards have been standardized. At present, most of standardized video coding processes are performed using hybrid video coding processes. In the hybrid video coding, reversible compression and irreversible compression are generally combined in order to obtain a desirable compression gain. Hybrid video coding is the basis for ITU-T standards (H.26x standards such as H.261 and H.263) as well as ISO/IEC standards (MPEG-X standards such as MPEG-1, MPEG-2, and MPET-4).
A video coding apparatus which executes hybrid video coding receives, as an input, a video signal representing an image sequence including a sequence of frames. In the hybrid video coding, each of the input images (frames) is divided into a plurality of blocks, and the image is coded on a per divided block basis. Among the divided blocks, the block having the largest size is referred to as a largest coding unit (LCU). For example in HEVC, the size of the largest coding unit LCU is 64×64 pixels. In addition, in H.264/MPEG-4 AVC, the LCU is further divided into coding units (CU) each having 16×16 pixels or so, and the image is coded on a per CU basis. In addition, the coding unit CU may be further divided into prediction units (PU) or transform units (TU) each having a smaller size. It is to be noted that the sizes of such blocks may vary depending on the kinds of the content of images. In addition, the coding schemes may vary depending on the blocks.
Since coding is executed on a per block basis as described above, when a coded bitstream is decoded, an image decoded therefrom may have a noticeable block boundary (a block noise). A significantly noticeable block noise appears when rough quantization is performed in a quantization process. Such a block noise adversely affects human visual recognition. In short, a block noise decreases image quality.
An exemplary method for reducing block noises is a method for filtering using a deblocking filter in the H.264/MPEG-4 AVC video coding standard or HM (HM is an HEVC test model in the trend report on video coding standardization, see Non-patent Literature 3). A deblocking filter is used for a reconstructed image to be referred to in a prediction process.